“Most and probably all of the distinctive infectious diseases of civilization have been transferred to human populations from animal herds.”

-William H. McNeill, Plagues and Peoples⁷⁴⁰

Domestication

Humanity’s biblical “dominion over the fish of the sea and over the birds of heaven; and every living thing that moved upon the earth” has unleashed a veritable Pandora’s ark full of humankind’s greatest killers.⁷⁴¹ Some diseases such as herpes and shingles seem to have always been with us, passed down the evolutionary chain. But most modern human infectious diseases were unknown to our hunter and gatherer ancestors.⁷⁴² Early humans may have suffered sporadic cases of animal-borne diseases such as anthrax from wild sheep or tularemia (“rabbit skinner’s disease”) from wild rabbits,⁷⁴³ but the domestication of animals triggered what the director of Harvard’s Center for Health and the Global Environment called the mass “spillover” of animal disease into human populations.⁷⁴⁴

Archeological evidence suggests that small, nomadic groups hardly suffered from contagious disease. With the advent of agriculture, though, communities settled and grew in relatively fixed locations, increasing their close exposure to their own waste and reservoirs of disease. Populations that domesticated only plants became more exposed to the few diseases they already harbored, but it was the domestication of animals that brought people in contact with a whole new array of pathogenic germs.⁷⁴⁵

Epidemic diseases tend to be harbored only by those animal species that herd or flock together in large numbers. This concentration allows for the evolution and maintenance of contagious pathogens capable of rapidly spreading through entire populations. Unfortunately, this same quality—the herd instinct—is what makes these animals particularly desirable for domestication. Domestication brought these animals once appreciated mainly from afar (along with their diseases) into close proximity and density with human settlements. As a zoonoses research team concluded, “The spread of microbes from animals to humans was then inevitable.”⁷⁴⁶

Tuberculosis, “the captain of all these men of death,”⁷⁴⁷ is thought to have been acquired through the domestication of goats.⁷⁴⁸ In the 20th century, tuberculosis (TB) killed approximately 100 million people.⁷⁴⁹ Today, tuberculosis kills more people than ever before—millions every year.⁷⁵⁰ The World Health Organization declared tuberculosis a global health emergency in 1993 and estimates that between 2000 and 2020, nearly one billion people may be newly infected. What started out in goats now infects one-third of humanity.⁷⁵¹

Domesticated goats seemed to have beaten domesticated cattle to the punch. Between 1850 and 1950, bovine tuberculosis, acquired mostly by children drinking unpasteurized milk, was responsible for more than 800,000 human deaths in Great Britain alone.⁷⁵² Interestingly, it can go both ways. The British Journal of Biomedical Science recounts that dozens of cases of bovine TB were traced back to a “curious farm-worker practice of urinating on the hay, perhaps on the folklore premise that the salts in urine are beneficial to the cattle.” Of course when it turns out the workers have genitourinary tuberculosis infections, it’s not so beneficial.⁷⁵³

Bovine tuberculosis continues to infect milk-drinking children to this day. In a study published in the American Academy of Pediatrics journal in 2000, doctors tested children with tuberculosis in San Diego and found that one-third of the tuberculosis wasn’t human. One in three of the children was actually suffering from tuberculosis caught not from someone coughing on them, but, the researchers suspect, from drinking inadequately pasteurized milk from an infected cow. The investigators conclude, “These data demonstrate the dramatic impact of this underappreciated cause of zoonotic TB on U.S. children.”⁷⁵⁴

Measles is thought to have come from domesticated cows, a mutant of the bovine rinderpest virus. The measles virus has so successfully adapted to humans that cattle can’t get measles and we can’t get rinderpest. Only with the prolonged intimate contact of domestication was the rinderpest virus able to mutate enough to make the jump.⁷⁵⁵ Though now considered a relatively benign disease, in roughly the last 150 years, measles has been estimated to have killed about 200 million people worldwide.⁷⁵⁶ These deaths can be traced to the taming of the first cattle a few hundred generations ago.⁷⁵⁷

Smallpox also may have been caused by a mutant cattle virus.⁷⁵⁸ We domesticated pigs and got whooping cough, domesticated chickens and got typhoid fever, and domesticated ducks and got influenza.⁷⁵⁹ The list goes on.⁷⁶⁰ Leprosy came from water buffalo,⁷⁶¹ the cold virus from cattle⁷⁶² or horses.⁷⁶³ How often did wild horses have opportunity to sneeze into humanity’s face until they were broken and bridled? Before then, the common cold was presumably common only to them.

New zoonotic infections from domesticated farm animals continue to be discovered. The 2005 Nobel Prize in Medicine was awarded to the scientists who discovered in 1982 that bacteria living in the human stomach, which they called Helicobacter pylori, caused stomach cancer and the vast majority of peptic ulcers worldwide.⁷⁶⁴ Roughly half of the world’s population is now infected.⁷⁶⁵ This ulcer-causing bacteria is thought to have originated in sheep’s milk, but is now spread person to person via oral secretions—saliva or vomit—or perhaps, like cholera, the fecal-oral route due to poor hand washing following defecation. What has become probably the most common chronic infection afflicting humanity,⁷⁶⁶ according to the CDC, came about because humanity started to drink the milk of another species thousands of years ago.⁷⁶⁷

A recent addition to the list of infectious farm animal bacteria is a cousin of H. pylori, known as Helicobacter pullorum (from the Latin pullus for “chicken”),⁷⁶⁸ infecting a large proportion of chicken meat. H. pullorum is thought to cause a diarrheal illness in people who contract it through the consumption of improperly cooked chicken fecal matter.⁷⁶⁹

Yet another newly described fecal pathogen, hepatitis E, is one of the latest additions to the family of hepatitis viruses. It can cause fulminating liver infection in pregnant women, especially during the third trimester, with a mortality rate of up to 20%. Scientists began to suspect that this virus was zoonotic when they found it rampant in North America commercial pork operations.⁷⁷⁰ Direct evidence of cross-species transmission was obtained in 2003.⁷⁷¹ Unlike a disease like trichinosis, which humans only get by eating improperly cooked pork, once a disease like hepatitis E crosses the species line, it can then be spread person to person. Between 1-2% of blood donors in the United States have been found to have been exposed to this virus.⁷⁷²